The NMDA receptor is arguably an important signaling mechanism in the human brain. The brain processes a complex array of information to allow humans to function, storing information from the past and analyzing this information in the context of the present to respond and plan for the future. These incredibly complex computations are mediated at the molecular level by the continual adjustment of the strength of synapses, the nodes for communication between nerve cells (estimated at about 60 trillion in the human brain).
Glutamate is the major excitatory neurotransmitter in the brain, utilized at 80% of these synapses. NMDA receptors are one of three classes that mediate synaptic transmission using glutamate. NMDA receptors play a critical role in regulating the strength of synapses, that is, in regulating synaptic plasticity. Thus, the NMDA receptor is at the molecular core of brain function, and in particular the cognitive functions of learning and memory. These facts underlie the tremendous therapeutic utility of modulating NMDA receptor function with new drugs to treat a broad range of neuropsychiatric disease and cognitive dysfunction.
The molecular basis of NMDA receptor function is increasingly well understood. The NMDA receptor is composed of four protein subunits, two NR1 subunits and two NR2 subunits. An NR1 subunit derived from a single gene is ubiquitously expressed throughout the brain and is common to all NMDA receptors. However, the four different NR2 subunits, NR2A-D, are derived from separate genes that are differentially expressed in different brain regions and by distinct populations of neurons within a particular region. Furthermore, individual neurons may express more than one NR2 subunit and individual NMDA receptors expressed by such neurons may contain two of the same NR2 subunits (for example, 2 NR2B subunits) or two different subunits (one NR2A and one NR2B subunit). Therefore, a drug that selectively modulates the activity of one NR2 subunit may do so at receptors that express two of the targeted subunits, or only one of the targeted subunits. Thus there is a need for new treatments for diseases related to the NR1/NR2B receptor.